1. Field of the Invention
The present invention relates to a fire-alarm system in which a fire receiver polls a plurality of terminal units such as relay devices and analog fire detectors.
2. Description of the Related Art
There is known a fire-alarm system of the so-called polling type in which a fire receiver polls a plurality of terminal units such as relay devices to which equipment to be controlled, including fire detectors, local bells and smoke protecting and discharging equipment, are connected, and analog fire detectors for outputting signals of physical quantities on detected fire phenomena (heat, smoke, gas, etc.); the terminal unit called by the polling sends status information such as the presence or absence of a fire signal and signals of physical quantities back to the fire receiver, and the fire receiver informs and/or indicates an outbreak of a fire based on the status information returned.
FIG. 5 is a circuit diagram showing one example of conventional fire-alarm systems of the type mentioned above. In the diagram, denoted by RE is a fire receiver, T1, T2, . . . are relay devices as one example of terminal units, and L1 is a pair of main signal lines connecting the fire receiver RE and the relay devices T1, T2, . . . to transmit signals and a source voltage. The terminal units may include analog fire detectors for outputting signals of physical quantities on detected fire phenomena, and other suitable equipment.
The fire receiver RE comprises a voltage signal transmitting circuit TX1 for transmitting a polling signal to the relay devices T1, T2, . . . , a voltage signal receiving circuit RX1 for receiving return signals sent back from the relay devices T1, T2, . . . , a microcomputer MPU1 for outputting the polling signal to the transmitting circuit TX1 and for decoding and discriminating the return signals received by the receiving circuit RX1, and a constant-current circuit SC for restricting a current flowing through the main signal lines L1. The transmitting circuit TX1 comprises a constant-voltage circuit SVH for an H (high-level) signal which outputs a voltage VH of 30 V, for example, a constant-voltage circuit SVL for an L (low-level) signal which outputs a voltage VL of 24 V, for example, and a switch SW turned on/off under control of the MPU1 for connecting or disconnecting the constant-voltage circuit SVH for H signal to or from the main signal lines L1, respectively.
The relay devices T1, T2, . . . each comprise a voltage signal receiving circuit RX2 for receiving the polling signal from the fire receiver RE, a voltage signal transmitting circuit TX2 provided with a switching circuit SWC for transmitting the return signal, a microcomputer MPU2 for decoding the polling signal received by the receiving circuit RX2 and for outputting the return signal to the transmitting circuit TX2, a Zener diode ZD for preventing the line-to-line voltage of the pair of main signal lines L1 from lowering below a predetermined voltage while the transmitting circuit TX2 is under signal transmission, and a constant-voltage circuit SV. The relay device T1 further comprises a fire signal detecting circuit FR for detecting a fire signal from a fire detector DE such as a heat sensor or a smoke sensor, and a local sound control circuit CC for controlling the sound of a local bell B.
In the above conventional fire-alarm system of the polling type, the number of the terminal units connected to the fire receiver RE, such as the relay devices T1, T2, . . . and fire detectors, depends on the location where the fire-alarm system is installed. The current flowing through the main signal lines L1 is controlled to be kept constant by the constant-current circuit SC on the side of the fire receiver RE, but the current flowing through the terminal unit under signal transmission varies with the number of the terminal units connected. Compare, by way of example, the case that the number of the terminal units connected is 200 and the case that the number is 100.
The comparison is made on condition that a constant current value (limit current) of the constant-current circuit SC in the fire receiver RE is set to 450 mA and a monitoring current per relay unit is 2 mA, for example.
In the case that 200 terminal units are connected to the fire receiver RE, the current consumed by the terminal units under signal transmission is given by: EQU 450 mA-(2 mA.times.200)=50 mA
In the case that 100 terminal units are connected to the fire receiver RE, the current consumed by the terminal units under signal transmission is given by: EQU 450 mA-(2 mA.times.100)=250 mA
Thus, the current consumed by the terminal units under signal transmission varies greatly with the number of the terminal units connected in spite of the fact that the current flowing through the main signal lines L1 is kept constant by the constant-current circuit SC provided in the fire receiver RE. In the above example, assuming that the current required for the terminal units during signal transmission is 50 mA, the terminal units consume an extra current of 200 mA in a signal transmitting state, when the number of terminal units connected thereto is 100. Furthermore, the signal detected by a signal detecting resistor R1 of the voltage signal receiving circuit RX1 in the fire receiver RE varies to a large extent depending on the number of the relay devices, fire detectors and other terminal units connected thereto, making it very difficult to design a circuit involving an amplifier, a comparator, etc. of the voltage signal receiving circuit RX1 or to set a signal discriminating level.
Meanwhile, the fire receiver RE transmits the polling signal to the terminal units such as the relay devices T1, T2, . . . and fire detectors by switching the voltage supplied to the main signal lines L1 between the voltage VH and the voltage VL.
When those voltages VH and VL are supplied to the terminal units, their voltage values are dropped due to a line resistance R0 of the main signal lines L1. Assuming that such a voltage drop is DV and a voltage threshold required for the voltage signal receiving circuit RX2 in each of the relay devices T1, T2, . . . to discriminate the H signal and the L signal is Vth, EQU (VH-DV)&gt;Vth
must be satisfied for enabling the relay device to discriminate the H signal and the L signal. In other words, the line resistance R0 cannot exceed a predetermined value. Assuming that the voltages VH, VL, Vth are respectively 30 V, 24 V, 26 V, by way of example, with 2 V taken as an allowance for signal discrimination, EQU VH-DV&gt;Vth+2=26+2=28
must be satisfied.
It is also assumed that 200 terminal units such as relay devices and analog fire detectors, each requiring a supervisory current of 2 mA, are connected like the above example. In this case, the following equations hold: EQU DV=R0.times.(0.002.times.200)=0.4.times.R0 EQU VH-DV=30-0.4.times.R0&gt;28
hence R0&lt;5(X)
Accordingly, in application to large-scaled fire-alarm systems (which include a large number of terminal units connected to the fire receiver, or which have the long main signal lines L1 ), it is required to reduce the current consumed by each of the terminal units, or raise the voltage VH, or make the line resistance R0 smaller, i.e., make the lines thicker, or lower the minimum operating voltage of the terminal unit. However, reducing the current consumed by the terminal unit or lowering the minimum operating voltage of the terminal unit has technical limitations. Also, it is difficult to achieve a smaller line resistance R0 with existing signal lines. Raising the voltage VH brings about a problem in that existing equipment can not be used.
In addition, conventional fire-alarm systems include no means for easily knowing whether the terminal unit actually receives the polling signal from the main signal lines L1, or whether the terminal unit actually transmits the polling signal to the main signal lines L1. Therefore, it is difficult to determine in the occurrence of an abnormal condition or which part has failed.